1. Field of the Invention
The present invention relates to a microwave electronic circuit and more specifically to a frequency-doubling voltage-controlled oscillator (VCO) having low frequency-modulation noise in the vicinity of the center frequency or so-called carrier frequency of the oscillator.
2. Description of the Prior Art
By definition, a voltage-controlled oscillator (VCO) makes it necessary to provide a resonant circuit which permits oscillation over a very wide range of frequencies corresponding to a flat curve of frequency as a function of voltage whereas an oscillator for a pure frequency has a very sharp curve with minimized frequency modulation (FM) noise in the vicinity of the carrier.
In point of fact, one of the shortcomings of VCOs is their excessive FM noise in the vicinity of the carrier. For example in the case of field-effect-transistor oscillators having a substantial bandwidth (1 octave), an FM noise level of -65 to -80 dBc/Hz at 100 kHz on each side of a 5-GHz carrier is a common value.
In order to reduce the FM noise in the vicinity of the carrier, it is a known practice to construct a VCO in the form of a frequency-doubling or so-called doubler oscillator in which two transistors are mounted in parallel and operate under conditions such that their drain currents are in phase opposition in the fundamental mode.